California Chrome, ridden by Victor Espinoza, sprinted to victory in the $10-million Dubai World Cup, beating Mubtaahij in the world's richest horse race.
Trained by Art Sherman, the 15-8 joint favourite won the 2,000-metre race by 3.75 lengths, despite Espinoza having to battle with a slipping saddle during the ride.
"He likes to run clear of his horses, he doesn't like to be bumped around. Victor knows him, he did a super job," trainer Sherman told reporters after the race at the 60,000-capacity Meydan racecourse.
Hoppertunity came third of the 12 runners in the contest.
"We've come across a superstar on the dirt, as California Chrome is - fantastic, and I am honoured to run second to him," Mubtaahij's trainer Mike de Kock said in televised comments.
A switch to a dirt track last year has helped to attract U.S. horses to the event. Eight of the 12-horse field were American this year.
De Kock told reporters that he was still optimistic about runner-up Mubtaahij, owned by Sheikh Mohammed Bin Khalifa Al Maktoum.
"He certainly is not a horse that I want to give a break to now, he's a horse that I want to keep on with," he said.
Chrome, who was the favourite last year but came a surprise second, has been in Dubai for more than two months, whereas last year he arrived just 10 days before the race.
The five-year-old is owned by California Chrome LLC and is also the 2014 Kentucky Derby and Preakness champion.